The invention relates to a method for the simultaneous preparation of carboxylic acid trimethylsilyl esters of the formula R-COOSi(CH.sub.3).sub.3 (I) and of carboxylic acid trimethylsilyl amides of the formula RCONHSi(CH.sub.3).sub.3 (IV) or N.N- and N.O-bis-trimethylsilylcarboxylic acid amides of formulas II and III, respectively: ##STR1## The N.O isomers of formula III are preferred. II and III were not separated. R represents preferably CH.sub.3, C.sub.2 H.sub.5 , C.sub.3 H.sub.7 , CH(CH.sub.3).sub.2 , CH.sub.2 .dbd.CH, CH.sub.2 .dbd.CCH.sub.3 , CH.sub.2 F, CHF.sub.2 or CF.sub.3 , but R generally can originate from carboxylic acid anhydrides of the formula (RCO).sub.2 O (V) which can be reacted with hexamethyldisilazane.
According to the state of the art, the acylation of hexamethyldisilazane to form substances of formula I (R.dbd.CH.sub.3) has already been attempted with ketene (Chem. Abstr. 57, 11224c), but it is difficult to control. The yield is impaired by undesired byproducts which prevent obtaining a pure and stable product.
With carboxylic acid chlorides, in the presence in some cases of an acid acceptor, the acylation is performed with an unsatisfactory yield and with the formation of many byproducts, especially with degradation to nitrile compounds.
The preparation of carboxylic acid trimethylsilyl amides RCONHSi(CH.sub.3).sub.3 (IV) has been performed heretofore from acid amides RCONH.sub.2 with hexamethyldisilazane (G. Schirawski and U. Wannagat, Monatshefte Chem. 1969, 100 (6), 1901-9). The special disadvantage in this case is the occurrence of byproduct having a triazine structure. Furthermore, unusable ammonia is undesirably released.
The conventional methods for the preparation of persilylated carboxylic acid amides of formulas II and III from carboxylic acid amides with trimethylchlorosilane and a tertiary amine as acid acceptor (L. Birkhofer and A. Ritter, Angew. Chem. 75, 93-4, 1963) causes the formation of very large amounts of salts of two equivalents of tertiary amine hydrochloride. Resorting to high dilution requires large amounts of solvents that have to be processed, limits the capacity, and leads to delayed reaction. In the case of incomplete transformation, contaminated products of poor shelf life are obtained.
Accordingly, the problem presents itself of finding an improved, simple method for the preparation not only of carboxylic acid trimethylsilyl esters but also of silylated carboxylic acid amides.